The present invention relates to reading information from tracks in a tape-drive system, and more particularly, to using combined soft detection/soft decoding in storage channels.
Reverse concatenation is a technique used to improve the robustness of tape-drive systems and increase tape cartridge capacity. In this technique, modulation encoding is performed prior to error-correcting code (ECC) encoding, so that the read channel is able to implement ECC decoding prior to modulation-code decoding. This approach allows for the possibility of applying soft ECC decoding techniques because soft reliability information extracted from channel data by a soft detector may be made available to the soft ECC decoder directly. Low-density parity-check (LDPC) codes are a family of codes that are capacity approaching, simple to encode, and attractive for efficient soft decoding. Therefore, they represent attractive candidates as component codes in a reverse-concatenation architecture for tape-drive systems.
In the read channel of such a reverse-concatenation architecture, a soft detector extracts soft reliability information on channel bits and passes this information to a soft LDPC decoder. The soft detection/decoding process involves two types of recursions or iterations: iterations within the LDPC decoder (the sum-product decoding algorithm—or variants thereof—performs iterations and exchange of soft information between so-called “check nodes” and “symbol nodes”) and iterations and exchange of soft information between decoder and detector. The implementation of such a soft detector/decoder poses significant challenges because it is desired to maximize system performance by increasing the number of iterations, which poses stringent requirements on circuit speed and chip area. In particular, the exchange and feedback of soft information between decoder and detector represents a significant bottleneck in the channel. Accordingly, a system and method that addresses this problem would be beneficial to tape-drive systems.